Perovskite phase mixed metal oxide ceramics are interesting materials due to changes in their physical properties on application of an external electrical stimulus. These properties include ferroelectric, pyroelectric, piezoelectric and dielectric behavior and have lead to numerous applications in electro-mechanic transducers, light modulation, charge storage, non-volatile memory applications and infra-red detection. (West, A. R., in "Solid State Chemistry and Its Applications", John Wiley and Sons, 1989. Meyers, E. R.; Kingon, A. I. eds, "Ferroelectric Thin Films", Materials Research Society: Pittsburgh, Pa., 1990, Vol 200. Bhalla, A. S.; Nair, K. M., Eds, "Ceramic Transactions: Ferroelectric Films", volume 25, The American Ceramic Society, Westerville, Ohio, 1992). Since these physical properties generall arise from the crystal chemistry of these materials, the formation of pure, stoichiometric, homogeneous, crystalline metal oxide films with controlled crystalline size is crucial. An additional problem that is commonly encountered in the formation of these materials is crystallization of the pyrochlore phase which does not exhibit the properties described above.
The different physcial properties related to the phase transitions are sensitive to the chemical composition, the purity, the number of surface and bulk defects, the grain size and the sintering conditions. The need to control these parameters is critical for the quality control of the devices produced from these materials. For example, the loss of Pb from lead titanate precursors during thermal processing is common due to the high volatility of PbO. This can be detrimental as any deviation from the correct stoichiometry will introduce TiO.sub.2 as an impurity which will deteriorate the ferroelectric properties of the product.
The physical properties of these materials are often tailored through formation of non-integral stoichiometry phases. The reason for producing these materials is to tailor the properties for the particular application. For example, BaTiO.sub.3 is not useful in its pure form because the high permeativity values are limited to a narrow temperature range near the Curie point at 130.degree. C. which is outside the temperature range for electronic applications. The goal of doping BaTiO.sub.3 is to lower the T.sub.C to room temperature and to broaden the temperature range over which the permeativity is high. To achieve this, partial substitution of Ca for Ba and Zr or Sn for Ti is used to give formulations of the type Ba.sub.1-x Ca.sub.x Ti.sub.1-x Zr.sub.x O.sub.3. By utilizing these substitutions the phase transitions which exist in pure BaTiO.sub.3 can be moved into a narrow band around room temperature to give a high permeativity under these conditions.
For the successful integration of these ceramics into silicon device technology, the problems of high crystallization temperatures (&gt;400.degree. C.) and kinetically slow crystallization must be overcome. Low crystallization temperatures are required to prevent the degradation of the underlying materials (especially aluminum) in the device structure. To achieve this goal a great deal of research has been carried out to prepare crystalline materials at low temperatures via thermal decomposition of metal-organic precursors. This route has the advantage that thin films can be spin-coated onto the silicon wafer and fired to give the crystalline ceramic film. However, no general process for the formation of metal oxides at low temperatures (&lt;400.degree. C.) has been identified.
In the past, a variety of methods for the preparation of these materials has been explored. The industrial routes to BaTiO.sub.3 powder involve the thermal reaction (800.degree.-1100.degree. C.) between BaCO.sub.3 and TiO.sub.2 or the thermal decomposition of BaTiO(C.sub.2 O.sub.4).sub.2.4H.sub.2 O. Hydrothermal synthesis of BaTiO.sub.3 powder has been achieved at much lower temperatures (150.degree.-200.degree. C.) by reaction between barium and titanium hydroxides (Vivekanandan, R.; Philip, S.; Kutty, T. R.; Mater. Res. Bull., 1986, 22, 99.) in strongly alkaline (pH&gt;12) solutions in an autoclave at &gt;5 MPa or from barium titanium acetate gels (Hennings, D.; Rosenstien, G; Schreinemacher, H.; J. Europ Ceram. Soc., 1991, 8, 107). However, the materials produced by this method exhibit some anomalous behavior thought to be derived from the incorporation of water and the presence of hydroxyl groups in the crystal lattice (Hennings, D.; Schreinemacher, H.; J. Europ Ceram. Soc., 1992, 9, 41.). Liquid phase chemical approaches to perovskite phase materials through metal-organic precursors have been extensively studied (Meyers, E. R.; Kingon, A. I. eds, "Ferroelectric Thin Films", Materials Research Society: Pittsburgh, Pa, 1990, Vol 200. Bhalla, A. S.; Nair, K. M., Eds, "Ceramic Transactions: Ferroelectric Films", volume 25, The American Ceramic Society, Westerville, Ohio, 1992.) and generally involve the reaction between metal alkoxides and metal carboxylates (Brinker, C. J.; Scherer, G. W. Sol-Gel Science, The Physics and Chemistry of Sol-Gel Processing, Academic Press, 1990. see e.g. "Better Ceramics Through Chemistry II, III and IV", Brinker, C. J.; Clark D. E.; Ulrich, Eds.) followed by hydrolysis and thermally-induced condensation/crystallization in the temperature range 500.degree.-700.degree. C. In addition, complicated and extended annealing schedules are often necessary to ensure complete crystallization of the correct perovskite phase. In some cases, annealing in the presence of an excess of lead vapor is necessary to prevent crystallization of the pyrochlore phase and/or loss of lead and crystallization of TiO.sub.2. Very few examples of the crystallization of perovskite phase materials at lower temperatures (&lt;400.degree. C.) exist, including BaTiO.sub.3 (50.degree. C., 12 hr. Mazdiyasni, K. S.; Dolloff, R. T.; Smith II, J. S., J. Am. Cer. Soc., 1969, 52, 523, and 100.degree. C., unspecified time, Larbot, A.; Garcia, F.; Guizard, C., Eur. J. Solid State Inorg. Chem., 1989, 26, 327, and PbTiO.sub.3, (375.degree. C., 10 hr.) Schwartz, R. W.; Payne, D. A.; Mater. Res. Soc. Proc., 1988, vol. 121, 199.). These processes are specific to these systems and cannot be extended to the formation of other metal oxide materials. In addition extended heating times were necessary to form crystalline material.
Accordingly, it is desirable to access this class of integral and non-integral stoichiometry perovskite phase metal oxide compounds by a generic method that overcomes the disadvantages of the prior art compositions and enables the formation of crystalline materials at low temperatures.